This invention relates generally to the data bus distribution apparatus and more particularly to the means for providing distributive electrical loading on a data bus having a variable number of peripheral devices such as would be encountered in a Local Area Network (LAN).
Data bus distribution among a number of peripheral devices is subject to electrical loading by the impedances of the peripheral devices which may be connected to the data bus. Ideally, the peripheral devices have an infinite real impedance which has no effect on a serial data bus of fixed characteristic impedance. Realizable peripheral devices, however, have a high (but not infinite) impedance which consists of both real and imaginary components. When a plurality of peripheral devices are connected in parallel on the data bus, the combined effect disturbs the characteristic impedance of the data bus resulting in improper data waveform transmission on the bus.
To correct this problem, specifications have been developed which define the maximum allowable impedance loading from a peripheral device and which specify that this loading should be distributed along the transmission line forming the data bus. One such specification is the Local Area Network ANSI/IEEE Std. 802.3-1985 ("Ethernet") which describes a means of attaching a plurality of devices to the local network medium via a multi-tap coaxial cable. The attachment of devices is made only at 2.5 m. intervals so that reflections from the non-infinite impedance of the devices do not add in-phase.
In order to realize a data bus distribution system which can accommodate a variable number of peripheral devices, it has been desirable to make the peripheral devices insertable and removable from the bus. This variability, however, causes the electrical distance between the peripheral devices on the data bus transmission line to change. In critical systems, this change can be unacceptable when a fixed transmission line technique is used. (Such a fixed transmission line technique has been employed in relatively tolerant analog multiplex systems such as that described in the Starplex.TM. CCIIT Multiplex Instruction Manual No. 68P81060E75-O available from Motorola, Inc., 1301 Algonquin Rd, Schaumburg, IL).
Those faced with similar problems in other technologies have run a copper foil trace forming a transmission line as part of a printed circuit board to each removable element and have not addressed the variable electrical distance between removable elements using the transmission line. Specifically, a multiple channel telephone multiplex system such as that shown in the above Starplex.TM. Multiplex Instruction Manual, pp. 1-9 (Equipment Shelves & Interconnect Boards Section), utilizes a copper foil trace to distribute a high frequency baseband among a variable number of "channel modems". A single "baseband BNC connector" assembly plugs into pins which protrude from the basis interconnect board and enable connection of the baseband to external circuitry. The electrical distance between these channel modems which are plugged into the interconnect board changes as the slots into which the channel modems are plugged change.